PM2.5 are particles 2.5 micrometres or less in diameter. PM2.5 is emitted from the combustion of fuels, such as wood and coal (eg from home heating and industry), and petrol and diesel (eg from vehicles). Natural sources have less influence on PM2.5 concentrations than PM10 concentrations. This means PM2.5 comes mainly from human activities. Nationally, burning wood or coal for home heating is the main source of PM2.5.

PM2.5 is a component of PM10 and is associated with similar health effects, ranging from respiratory irritation to some forms of cancer. However, the smaller PM2.5 particles are more closely associated with severe health problems.